1. Field of the Invention
This invention relates to curable epoxy resins. More particularly, to a novel curing accelerator.
2. Prior Art
Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with certain desirable properties. One such class of curing agents are generally the amines. The most commonly used amine curing agents are aliphatic amines such as diethylenetriamine, triethylenetetramine and the like and/or polyoxyalkylene polyamines; such as polyoxypropylenediamines and triamines.
Epoxy resin compositions having improved physical properties are obtained by employing polyoxyalkyleneamines, and polyoxyalkylenediamines in particular, as curing agents. It is common to employ with such epoxy resin compositions a co-curing agent such as those described in U.S. Pat. No. 3,549,592.
Also known to be effective as epoxy curing agents or co-curing agents are various ureas and substituted ureas, such as those disclosed in U.S. Pat. Nos. 3,294,749, 2,713,569, 3,386,956, 3,386,955, 2,955,372 and 3,639,338. The ureas disclosed in the above references are useful as either curing agents or as curing accelerators.
Aliphatic or aromatic compounds having a single terminal ureido group are well known. It has been disclosed in U.S. Pat. No. 2,145,242 to Arnold that diureido terminated aliphatic compounds can be produced by reacting an aliphatic diamine wherein each terminal amine has at least one labile hydrogen with urea. Other substituted ureas are disclosed in U.S. Pat. No. 3,965,072.
Diureide terminated polyoxyalkylene material provides cured epoxy resin compositions exhibiting outstanding strength of adhesion as disclosed in co-pending application Ser. No. 743,814 filed Nov. 22, 1976.
In several applications, accelerated curing of epoxy resins at ambient temperatures is necessary. A typical example is the use of an epoxy resin as an adhesive in a flammable environment. Curing with many prior art amines may be too slow for this and other specific situations.
My U.S. Pat. Nos. 3,875,072 and 3,943,104 disclose an accelerator comprising piperazine and an alkanolamine. This accelerator combination has one disadvantage. It tends to crystallize at low temperatures, thus reducing its usefulness in some cases.
The present invention is an accelerator which does not crystallize at low temperatures.